The present invention relates to pulsed or intermittently operated crossed-field devices such as crossed field amplifiers (CFAs). By way of example, U.S. Pat. No. 3,255,422 shows one such CFA device wherein a microwave entry waveguide provides radio frequency energy to an entry port for a slow wave propagating structure on an anode. A cathode is opposed to this anode across a gap. A solenoid provides a strong magnetic field perpendicular to the applied electric field. The cathode is formed of a material having a secondary emission ratio greater than unity so that electrons emitted from the cathode due to the electric field follow re-entrant trajectories in the magnetic field and bombard the cathode to cause further electron emission. Energy exchange between the emitted electrons and the rf field results in amplification of the input signal, which is then coupled through an outlet port as an amplified signal into a waveguide.
In some applications, a CFA is designed to operate with the anode and cathode always energized, and a pulsed input signal is applied to the inlet port and amplified as it travels to the outlet. In that case, because the cathode is formed of a material selected to readily emit secondary electrons, such devices, if not provided with a means for shutting down the electron emission, may continue to run spontaneously even when the input signal is removed. Thus, for those devices, it is customary to provide a control electrode as set forth in the aforesaid U.S. Pat. No. 3,255,422 which during a turn-off phase is pulsed near anode potential to capture electrons and thus end the secondary electron re-emission. Other methods of assuring shut-down for these pulsed-input devices may utilize specialized constructions, such as the second anode structure described in commonly-owned U.S. patent application Ser. No. 09/259,643. Other applications of CFAs operate by switching the anode/cathode potential on and off. These latter CFAs operate without such a control electrode structure, but are subject to multipactor effects from the high voltage switching cycles.
In general, when these devices are operated for example as high-power radar tubes, they are operated with extremely high powers but low duty cycles, thus requiring them to be switched on and off at high speeds. The nature of the crossed-field amplifier, requiring large secondary electron emissions, makes it particularly susceptible to multipactor and other gas discharge effects during such switching, and the multipactor effects are particularly pronounced with lower magnetic fields.
Some potential advantages of using a low magnetic field have been recognized since the early days of magnetron and crossed-field amplifier development. However, in general, for high power amplifier as described above, it has been necessary to employ relatively high magnetic fields in order to avoid multipactor and sustain clean operation and switching. The field strength is typically several thousand Gauss in the interaction region of an S-band device operating at 150 to 300 kilowatts peak power, and somewhat lower at the surfaces and lead-in portions of the amplifier outside the interaction space where many multipactor effects are sustained. The ability to operate a high powered microwave or millimeter wave device with a lower magnetic field would offer advantages in terms of physical weight as well as operation and signal quality.
The presence of multipactor and gas discharge effects on the anode circuit and elsewhere within the device has been recognized as a significant problem arising with a low magnetic field. One existing theory suggests an approach to overcome this problem by simply employing a magnetic field which exceeds the cyclotron value to suppress discharges perpendicular to the magnetic field (i.e., between anode vanes). However, applicant""s initial attempt to implement such a solution with a magnetic field operating at a strength slightly above the cyclotron value showed that while discharges between vanes exist mainly below the cyclotron resonance field, they extend above this value to an extent which can interfere with the performance of the cross-field amplifier. The physics of this process is complex. Ordinarily, secondary emission occurs on a time scale orders of magnitude less than an rf cycle. The cyclotron resonance multipactor cut-off depends on this property. Both analysis and experiment suggest that the discharges above the cyclotron resonance field result from an ability of oxidized surfaces with insulating properties to store charge generated by incident primary electrons for an appreciable fraction of an rf cycle, and to subsequently emit this charge when the rf field reverses. The emission is a form of thin-film field emission, but its result is the same as, or may be modeled as, a secondary emission with time delay having a duration which is significant relative to the duration of an rf cycle. Reference to this effect in the literature employs the term xe2x80x9ctime-delayed secondary emissionxe2x80x9d. Unfortunately, oxides are necessary components to achieve the high yield of secondary electrons upon which the traveling wave amplification effects depend, and the harsh conditions within the interaction space operate to transport these, or produce other, oxides. Given the presence of such oxides, it would appear that, at present, only very high magnetic fields can effectively eliminate multipactor.
Accordingly, it would be desirable to provide such a device that operates dependably without multipactor at lower magnetic field strengths.
The present invention overcomes deficiencies of known devices by providing a crossed-field device such as a crossed-field amplifier or magnetron wherein a distributed cathode body is spaced from an anode to provide an electric field in a slow wave region or interaction space, which, in a crossed-field device, extends for example between a signal inlet and an outlet, and the slow wave region is arranged to have a magnetic field oriented perpendicular to the electric field to enhance generation of secondary electrons. At least one window, and in an amplifier preferably both of the output and input windows to the amplifier tube, are formed with a closely spaced set of grooves over the window surface, and at least one other major surface component of the assembly is grooved, coated or is otherwise configured to further inhibit secondary electron emission or delayed emission. In a preferred embodiment the windows are formed of ceramic or material transparent to radiation, and have a regular array of parallel grooves approximately ten mils wide and ten mils deep on a ten mil spacing. The amplifier may possess a cathode of secondary emitting material, such as beryllium, and secondary emission may be inhibited on the anode surface by providing a molybdenum oxide coating. Alternatively, or in addition, the cathode may have molybdenum wires, strips or segments included in a portion of its surface area to provide a reservoir or source of material that supplies the anode with sputtered or vaporized molybdenum during operation to maintain its operating characteristics. The grooved window may also be employed in conjunction with grooving of some or all of the major surfaces of the input and/or output transformers.